In fluid conveying systems which utilize plastic pipe, including natural gas distribution networks, segments of plastic pipe must be coupled with fittings capable of providing a reliable seal over a range of temperatures and pressures. Providing a reliable coupling for plastic pipe is difficult due to problems, such as stress, that cause fatigue, stretch, or failure of the plastic pipe and that result in leaking at the coupling from an inadequate seal. Couplings used in plastic pipe distribution systems must be able to withstand axial stresses, such as those caused by temperature induced length variations, without a decrease in their sealing capabilities. In addition, plastic pipe couplings should be easily connectable by ordinary workers, preferably without being totally disassembled in preparation for connection.
Plastic pipe couplings are currently available in many different functional configurations. The typical coupling includes a body portion into which an end of the plastic pipe is inserted. Generally, the coupling includes a metal tube which fits inside the plastic pipe to act as a stiffener for the pipe in the area where the pipe is gripped and sealed by the coupling. Inside the body portion of the coupling, and encircling the plastic pipe, are one or more elastic sealing rings or gaskets, one or more washers, and a grip ring for securing the plastic pipe in the coupling. A threaded nut surrounds the plastic pipe and engages corresponding threads on the body portion of the coupling. Tightening the nut with respect to the body portion forces the grip ring to compress radially inward to grip the plastic pipe, and either the grip ring, an extension of the nut, or both provide pressure to compress the elastic gasket to form a seal between the exterior surface of the plastic pipe and the interior of the body portion of the coupling.
Typical coupling designs utilize a grip ring that must be expanded radially during insertion of the plastic pipe so as to fit around and grip the pipe. This characteristic makes insertion of the pipe more difficult and creates a locking condition that prevents withdrawal of the pipe except by disassembling the coupling, even though the nut has not been tightened.
In couplings where the compression of the elastic gasket is provided solely by axial movement of the grip ring, it is possible for the grip ring to seize the pipe before the nut is completely threaded on the coupling. In this case, further axial advancement of the grip ring is prevented, and an inadequate seal may result due to low compression of the elastic gasket. Also, in cases where a proper seal has been established initially by action of the grip ring against the elastic gasket, a stretching of the pipe due to tensile stress may reduce the compression of the gasket and thus effectiveness of the seal.
In couplings where the elastic gasket is compressed solely by the action of the nut, the gasket can be fully compressed before the grip ring effectively grips the pipe. This problem can occur in couplings that provide a fixed-volume recess for the gasket between a body shoulder and a rigid washer and/or that provide a stop to limit tightening of the nut. Furthermore, fixed-volume recesses and stops that limit compression of the gasket can result in undercompression of the gasket due to dimensional variations from manufacturing tolerances of the pipe and components of the coupling.
Thus, there is a need for an improved coupling for plastic pipe that is easy to connect and that provides a reliable seal, even with component dimensional variations and/or stretching of the pipe due to axial stress.